In the development of variable speed sheaves wherein both flanges are adjustable relative to a common sleeve component, several problems exist which seriously limit sheave design. In one form of variable speed sheave currently on the market, first and second adjustable flanges have their hubs telescoped with the interior of the two hubs bearing directly on the sleeve. As the pitch diameter of the sheave is changed, the hubs of the two flanges slide axially relative to each other and relative to the sleeve. This arrangement has several serious drawbacks, as follows:
(1) The minimum pitch diameter of the sheave cannot be as low as for a single adjustable flange sheave. This necessitates larger diameters to obtain the required speed change ratio and necessarily increases cost. PA1 (2) Centering of the flanges becomes a costly problem. It is difficult to accomplish centering with two springs, because the bearing loads of one adjustable flange are greater than for the other flange, causing unequal drags as the pitch diameter of the sheave is changed, and causing the belt center line to shift. Mechanical centering devices to deal with this problem are known but all are quite costly and are often sources of fretting due to minute movements in the absence of lubrication. PA1 (3) The sheave is more costly to produce because the two flange elements are non-identical. Furthermore, tight tolerances must be maintained between the annular hubs of the two flanges to guarantee proper flange concentricity.
In a second form of sheave commonly in use, the sheave is symmetrical about the belt center line allowing the use of common flanges, springs and end caps. However, the distance from the center line of belt pull to the bore end of the sheave must be minimized to satisfy motor bearing specifications. This requires short adjustable flange hub lengths and, therefore, high bearing stresses. These high bearing stresses, in turn, produce a high rate of wear of the bearing surfaces and relatively short sheave life.
If the bearing surfaces are not separated by lubricant, but instead are boundary (dry) lubricated bearings, it becomes extremely difficult to provide adequate sheave life when bearing pressures are high, i.e., 300-400 psi. If the bearing pressure can be reduced, bearing life can be markedly extended and the bearing cost is reduced.
The present invention has for its objective to improve upon the prior art pertaining to dual adjustable flange variable speed sheaves by providing a dry lubricated sheave with greatly reduced bearing pressures and several other unique features which will become apparent during the course of the following description.